This application claims priority from Korean Patent Application No. 10-2004-0072087, filed on Sep. 9, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates to an optical system having an element for correcting mirror aberrations, and an optical pickup apparatus employing the same, and more particularly, to a an optical system in which aberrations generated due to an error in the surface shape of a mirror are corrected using a hologram optical element, and an optical pickup apparatus employing the same.
2. Description of the Related Art
FIG. 1 illustrates the configuration of a typical optical pickup apparatus. As shown in FIG. 1, an optical pickup apparatus 100 comprises a light source 110, a mirror 120 that is an optical path changer, an optical path separation member 130, an objective lens 140 and a photodetector 150. The light source 110 emits light that is reflected by the mirror 120 to be incident on the optical path separation member 130. The optical path separation member 130 comprises a polarization diffraction element 132 that either linearly transmits or diffractively transmits incident light according to the polarization state of the incident light, a quarter wave plate (QWP) 134 changing the state of the polarization of incident light, and coating layers 131, 133, 135 for protecting the polarization diffraction element 132 and the QWP 134. According to the above configuration, the light reflected by the mirror 120 transmits the optical path separation member 130 as it is and proceeds further. However, the light reflected by an optical disc D is diffracted at a predetermined angle by the optical path separation member 130. Thus, the light reflected by the mirror 120 does not proceed toward the light source 110 and is received by the photodetector 150.
Conventionally, in manufacturing an optical pickup apparatus, the respective optical parts are separately manufactured and then assembled into an optical pickup apparatus. However, as the need for mass production of compact optical pickup apparatuses increases, an integrated optical pickup apparatus has been developed as one of methods to remove difficulties in an assembly process. The integrated optical pickup apparatus is generally manufactured by combining a light emitting portion, a light receiving portion and an optical path separating portion, except for the objective lens that is a light collecting portion, into an optical bench, and integrating the optical benches. As a result, the assembly process is simplified and the mass production of optical pickup apparatuses can be made easy.
FIG. 2 illustrates an example of an integrated optical pickup apparatus. As shown in FIG. 2, an integrated optical pickup apparatus 200 comprises a mirror bench 210, a light receiving bench 220, an optical path separation portion 230 and an objective lens 240. A groove 211 is formed in an upper surface of the mirror bench 210 and a first inclined mirror 212 and a second inclined mirror 215 are formed at opposite ends of a bottom surface 213 of the groove 211. A light source 222 for emitting light toward the second inclined mirror 215 and a photodetector 221 for receiving light reflected by the first inclined mirror 212 are installed at a lower surface 225 of the light receiving bench 220. Also, the optical path separation portion 230 and the objective lens 240 are installed at a position facing the second inclined mirror 215. The optical path separation portion 230 comprises, as described above, a polarization diffraction element 231 that linearly transmits or diffractively transmits incident light according to the polarization state of the incident light and a quarter wave plate 233 changing the state of the polarization of incident light.
The mirror bench 210 is manufactured by wet etching silicon according to a general semiconductor process. However, when a mirror is manufactured in a wet etching method, the etching is not uniform. As shown in FIG. 3, the surface of the mirror is uneven, which generates aberrations such as astigmatism. In particular, this problem is severe in the case of an optical pickup apparatus comprising a large numerical aperture (NA) that uses a short wavelength according to the high capacity of a storage medium. The mirror used in the optical pickup apparatus requires a peak-to-valley (PV) value to be not more than ⅙ of a wavelength. When such a condition is applied to a reference wavelength of a blue-ray disc that is 407 nm, the PV value must be not more than 67.8 nm. However, in the case of a mirror manufactured in the wet etching method, since the PV value (or waviness) is about 150-250 nm, the above condition is difficult to meet. Although a conventionally manufactured mirror that is polished after molding can meet the required PV value, the conventional method is not appropriate for the mass production of integrated optical pickup apparatuses and is costly.